Supermarine Spitfire F.21

By early 1942 it was clear that a Spitfire behind a two-stage Griffon needed more than local reinforcement: at the speeds it could reach, the original wing came uncomfortably close to aileron reversal, theoretically around 930 km/h. Supermarine rebuilt the structure and skinning, gained 47 per cent in torsional stiffness and pushed the theoretical reversal speed to 1,328 km/h. The ailerons grew five per cent, hung on continuous piano hinges and were extended by eight inches, straightening the trailing edge: for the first time a Spitfire wing was no longer truly elliptical.

The five-blade propeller of 3.4 m was larger than the Mk XIV's and forced longer undercarriage legs to keep the blades clear of the ground. Since the longer legs no longer fitted their bays, a system of levers shortened them as they retracted and extended them on the way down. The track widened by nearly eight inches, improving ground handling, and the wheels were fully enclosed by triangular doors. Armament was standardised as four 20 mm Hispano cannon with 150 rounds per gun and no machine guns at all.

The first true prototype, PP139, flew in July 1943 and the first production aircraft, LA187, on 15 March 1944. In the air the Mk 21 proved acutely sensitive to trim changes, and trials through late 1944 and early 1945 produced a verdict that became famous: the aircraft was not dangerous, but it was unlikely to prove a satisfactory fighter and no further attempt should be made to perpetuate the Spitfire family. With Castle Bromwich already converted to build it, Supermarine took the criticism seriously. Revised trim-tab gearing and a series of subtle control changes were enough: by March 1945 a fresh report cleared the Mk 21 for instrument and low flying and judged it suitable for the average pilot.

91 Squadron received its first Spitfire 21s in January 1945 with too little war left to meet the Luftwaffe in strength; its oddest success came on 26 April 1945, when two aircraft strafed and claimed the sinking of a German midget submarine caught on the surface. With the war over almost every order was cancelled and only 120 were completed. In 1946 forty were delivered to Shoeburyness, where their leading edges were stripped off and destroyed in lethality trials; some had flown less than five hours.